Hydraulic control system for extensible crane

ABSTRACT

A hydraulic control system for producing synchronized extension and retraction of at least three crane sections which includes a first cylinder and piston rod assembly with the cylinder being attached to an intermediate section of the crane and the piston rod connected to a first or fixed crane section. A second cylinder is connected to the intermediate section and its piston rod is attached to a third section with a fluid divider supported on the second section and having first and second outlet ports, respectively, connected to one end of each of the cylinders and an inlet port. A first flow passage means extends through the first cylinder and piston rod assembly and is connected to the inlet port of the flow divider with a second flow passage means extending through the first piston rod and communicating with the opposite end of the first and second cylinders so that a pressured fluid source connected to one flow passage and a reservoir connected to the other flow passage means will extend the assemblies and the crane sections with the flow divider means producing synchronized movement of the crane sections. The control circuit further includes means for blocking flow from one end of each of the cylinders when the pressured fluid in the first fluid passage means is below a predetermined level. In addition, the flow divider means includes pressure responsive check valves providing unidirectional flow in opposite directions between the two outlet ports so as to interconnect the two ports when the pressure in either of the outlet ports exceeds a predetermined level.

United States Patent Wirkus [451 Apr. 25, 1972 HYDRAULIC CONTROL SYSTEMFOR cludes a first cylinder and piston rod assembly with the EXTENSIBLECRANE cylinder being attached to an intermediate section of the I craneand the piston rod connected to a first or fixed crane Inventor! J p iSchofield, section. A second cylinder is connected to the intermediatesection and its iston rod is attached to a third section with a [73]Ass'gnee' Cm Company fluid divider sugported on the second section andhaving first 22 Fi J ly), 1970 and second outlet ports, respectively,connected to one end of r each of the cylinders and an inlet port. Afirst flow passage PP'- Nod 531848 means extends through the firstcylinder and piston rod assembly and is connected to the inlet port ofthe flow divider with a second flow passage means extending through thefirst [52] US. Cl ..9l/412, 21922//5l55,22 19220114148, piston rod andcommunicating with the pp end of the 5 l I Cl Flsb 11/16 first andsecond cylinders so that a pressured fluid source con 1 nected to oneflow passage and a reservoir connected to the Fleld -v "91/411 412; 92/1other flow passage means will extend the assemblies and the 92/52;212/55 144 crane sections with the flow divider means producing asynchronized movement of the crane sections. The control [56] Referen sCiied circuit further includes means for blocking flow from one end ofeachof the cylinders when the pressured fluid in the first UNITED STATESPATENTS fluid passage means is below a predetermined level. ln addi- 3386 594 6/1968 Grove ..2l2/l44 the divide means includes Pressure 48912/1969 Stauffer n check valves providing unidirectional flow inopposite 3 572 517 3/1971 Liebherr et a1. ..212/144 di'ecticns betweenas Primary Examiner-Edgar W. Geoghegan Attorney-Dressler, Goldsmith,Clement & Gordon [57] ABSTRACT the two ports when the pressure in eitherof the outlet ports exceeds a predetermined level.

6 Claims, 4 Drawing Figures HYDRAULIC CONTROL SYSTEM FOR EXTENSIBLECRANE BACKGROUND OF THE INVENTION The present invention relatesgenerally to cranes, and more particularly to an improved hydrauliccontrol system for extending and retracting boom sections of suchcranes.

Mobile cranes generally consist of a vehicle which has a tumable base orsupport rotatable-about a vertical axis on the frame of the vehicle. Acrane boom is generally pivoted about a horizontal axis on the supportand is raised and lowered in order to be capable of reaching variouselevations while rotation of the base permits movement of the boom toany position within the circle'of operation of the crane. In addition,it has become customary in recent years to increase the versatility ofthe machine by forming the boom of a plurality of extensible andretractable sections. Generally, the sections are of hollow polygonalconfiguration and are nestable within each other in the retractedposition. The extension and retraction of the respective sectionsrelative to each other, in present day cranes, is generally accomplishedthrough the use of fluid rams having respective elements connected tothe adjacent boom sections so that extension and retraction of the ramwill cause extension and retraction of the two sections.

In at three section crane boom, it has been customaryto attach thecylinder of a first fluid ram to the first section, which is pivoted onthe boom support and to connect the piston rod of the first fluid ram tothe second or intermediate boom section. Asecondfluid ram has itscylinder connected to the intermediate or second boom section and itspiston rod connected to the outer or third boom section. The respectivefluid rams are extended and retracted relative to each other byconnecting opposite ends of the cylinders of the respective fluid ramsto a source of pressured fluid and a reservoir through conduits orhoses. One such arrangement is disclosed in Stauffer, U.S. Pat. No.3,481,489.

One of the difficulties with an arrangement of this type is that thesecond cylinder of the second fluid ram moves with the second orintermediate section which necessitates com plicated mechanism forproviding adequate storage for the hydraulic conduits or hosesinterposed between the second fluid ram and the source of fluid pressureas well as the reservoir. One type of such arrangement is disclosed inthe above mentionedStaufier patent.

Suchan arrangement is not only expensive to construct but also requiresa considerable amount of maintenance due to the fact that the hoses aresubjected to a considerable amount of wear and, in many instances, maybe ruptured because of accidentally being trapped between relativelymovable elements.

BRIEF SUMMARY OF THE INVENTION The present invention may be summarizedas a hydraulic control system for extending and retracting a pluralityof boom sections located in telescoping relation to each other and inwhich the various parts of the hydraulic control system are designed andarranged in a manner to eliminate the need for providing a large amountof hose and a take-up mechanism for storing the hose during extensionand retraction of the boom sections relative to each other. Statedanother way, the present invention contemplates a hydraulic controlsystem for a plurality of boom sections in which both cylinders areattached to the same boom section and pressured fluid is delivered tothe boom section through variable length flow passages. This arrangementeliminates the need for providing hoses from the base section for theboom to the second section in its extended condition. In addition, thepresent invention contemplates dividing, the pressured fluid receivedat. the second boom section equally between the two fluid rams so as tocause synchronized movement of the boom sections relative to each other.

More particularly, the present invention contemplates a hydrauliccontrol system for a boom having a plurality of sections and one of thesections being supported on a substantially fixed support with a firstcylinder and piston rod assembly having its piston rod connected to thefirst boom section and it cylinder connected to a second boom section. Asecond cylinder and piston rod assembly has its cylinder connected tothe second boom section and its piston rod connected to a third boomsection. Pressured fluid is directed from a source through the firstfluid ram to a flow divider supported on the second boom section whichdistributes the fluid equally to one end of both of the cylinders Theopposite ends of the two cylinders are interconnected by a conduit and asecond flow passage means extends through the first fluid ram andcommunicates with the conduit. In addition, the flow dividerincorporates first and second pressure responsive relief valvesproviding unidirectional flow in opposite directions between the twooutlet ports of the flow divider with the respective relief valves beingresponsive to pressure of the fluid in the respective conduits toproduce a flow passage between the two outlets for the valve when thepressure in either outlet exceeds a certain level.

The hydraulic control system further includes safety means in theconduit between the two cylinders and in the first flow passage forblocking flow therein when the pressure in the second fluid passagedrops below a predetermined level.

The first and second fluid flow passages are incorporated entirelywithin the first fluid ram thus completely eliminating the need for anyflexible hosing or any relative movement between the cylinders of thefluid rams during extension and retraction of the crane sections.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS FIG. 1 illustrates aside elevation view of a mobile crane embodying the present invention;

FIG. 2 is an enlarged sectional view of one of the two fluid ramsforming part of the hydraulic control system of the present invention;

FIG. 3 shows the arrangement of parts of the hydraulic con trol systemin relation to the crane; and

FIG. 4 is a schematic illustration of the hydraulic control system ofthe present invention.

DETAILED DESCRIPTION While this invention is susceptible of embodimentin many difierent forms, there is shown in the drawings and will hereinbe described in detail one specific embodiment, with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit theinvention to the embodiment illustrated.

FIG. 1 of the drawings discloses a mobile crane, generally designated bythe reference numeral 10. The mobile crane consists of a frame 12supported on wheels 14 and having outriggers 16 connected thereto whichare extensible to raise the frame and the wheels above the ground levelin order to provide a fixed frame. A turntable 18 is defined on theupper end of the crane and supports a boom base or support 20 forrotation about a vertical axis.

The crane 10 further includes a boom 22 pivoted at one end on the baseor support 20 by a pin 24. Pivotal movement of the boom 22 relative tothe base 20 is accomplished by a fluid ram 26 interposed between theboom and the base.

As is customary in many cranes of this type, the boom 22 consists of aplurality of boom sections 30, 32 and 34. The three boom sections are ofgenerally polygonal hollow configuration and are mounted in telescopingrelation to each other. A load supporting member 36 is normallysupported on the outer end of the outer tubular section 34 and isadapted to be moved relative thereto by a winch and a cable (not shown).

As was indicated above, it has become customary to extend and retractthe three sections of the boom relative to each other through a fluidhydraulic control system which includes a fluid ram interposed betweeneach pair of adjacent sections.

With an arrangement such as this, in extending the second and thirdsections relative to each other, it becomes necessary to provideflexible hosing of sufficient length to reach the outer end of theintermediate section in its extended condition. During the retraction ofthe intermediate section within the inner section, a take-up mechanismmust be provided for storing the hosing in a manner that the hosingwould readily be extended upon subsequent extension of the two sections.The problem becomes even more complicated when the boom consists of fouror more sections.

According to the present invention, the problems of storing flexiblehosing during extension and retraction of boom sections relative to eachother is eliminated simply by an arrangement of parts which allows allof the connections to be substantially fixed relative to each other.This is accomplished by utilizing a cylinder and piston rod arrangementhaving internal flow passages extending therethrough and the piston rodof the first cylinder is connected adjacent the pivotal connection ofthe boom to its support while the cylinder of the assembly is located inthe second section. The second cylinder and piston rod assembly forextending and retracting the second and third sections has its cylinderdisposed in the same section as the first cylinder and connected theretowith the piston rod being connected to the subsequent section. With thisarrangement, all of the fluid for extending the piston rods is directedto the second boom section and is divided between the two cylinders inthe second section to simultaneously extend the two assemblies insynchronism with each other. During retraction of the boom sections, allof the fluid forced from the cylinders passes through the piston rod ofthe first cylinder assembly to the reservoir.

The arrangement of parts in the hydraulic control system for eliminatingthe take-up mechanism heretofore necessary for the hosing to the secondcylinder is shown in FIG. 3. The system includes a first fluid ram orcylinder and piston rod assembly 40 having its piston rod 42 connectedat 44 to the first boom section 30 at a location adjacent the pivot pin24. The cylinder 46 of the first fluid ram is trunnion mounted at 48 onthe intermediate boom section 32.

The hydraulic control system or circuit further includes a second fluidram or piston rod and cylinder assembly 50 having its cylinder 52trunnion mounted at 54 on the intermediate boom section 32 while thefree end of the piston rod 56 is connected at 58 to the third boomsection adjacent its outer end. With this arrangement, the fluidcylinders 46 and 52 are in adjacent relationship to each other and areconnected to the same boom section of the crane.

The hydraulic control system of the present invention further includesfirst and second flow passage means in the first fluid ram 40 fordirecting fluid to the second boom section to and from opposite ends ofthe respective cylinders 46 and 52. The fluid ram 40 may be of the typedisclosed in Antos et al. US. Pat. No. 2,787,383. To provide the twofluid flow passages through fluid ram 40, the piston rod 42 of the fluidram 40 consists of an outer tubular member 60 and (FIG. 2) an innertubular member 62 arranged in concentric relation to each other todefine an annular chamber 64 running substantially the entire length ofthe piston rod 42. One end of the annular chamber 64 is closed by piston66 fixedly secured to the ends of the tubular members 60 and 62 with thepiston 66 being slidably disposed within the cylinder 46. The oppositeend of the annular chamber 64 is closed by a plug 68 having an eye 70 onthe end thereof for connecting the piston rod 42 to the first boomsection 30. The plug 68 has an opening 72 communicating with the openingin the inner tubular member 62.

The fluid ram 40 further includes a tubular member 74 retained in anopening 76 in the head end of the cylinder 46 with the opposite end ofthe tubular member extending through an opening 78 in the piston 66 andtelescopingly received in the tubular member 62. Thus, the tubularmembers 62 and 74 cooperate to define a first fluid flow passage means79 extending through the piston rod 42 and the cylinder 46 which definefirst and second elements for the fluid ram 40. The annular chamber 64defines the second fluid passage means extending through the piston rodor first element of the fluid ram, with the inner end of the annularmember or second fluid passage means 64 communicating with one end ofthe cylinder 46 through a plurality of openings 80 (only one beingshown) in the outer tubular member.

The hydraulic control system further includes a flow divider valve ormeans 84 (FIGS. 3 and 4) supported on the second section 32 of the boom22 and connected to the first flow passage means 79 through a conduit86. The flow divider means is more clearly shown schematically in FIG. 4and includes an inlet port 88 having the conduit 86 connected theretoand first and second outlet ports 90 and 92. The first outlet port 90 isconnected to the head end'of the first cylinder 46 through a conduit 94while the second outlet port 92 is connected to the head end of thesecond cylinder 52 through a conduit 96. First and second restricters 97and 98 are located between the inlet port 88 and the respective outletports 90 and 92 and the valve is constructed in such a manner that fluidreceived in the inlet port is equally divided between the two outletports. With this arrangement, the fluid supplied through the firstpassage means 79 will be equally distributed to the head ends of therespective cylinders or elements 46 and 52 and will cause synchronizedextension of the respective piston rods or elements 42 and 56 relativeto the respective cylinders thereby producing synchronized extension ofthe three boom sections relative to each other.

The interconnection between the two cylinders 46 and 52 further includesa conduit 100 placing the rod ends of the respective cylinders incommunication with each other so as to connect the second fluid passagemeans to the rod ends of the respective cylinders.

The hydraulic control system further includes a reservoir and pump 112connected to a conventional valve 114, which in turn is connectedthrough conduits 116 and 118 to the respective fluid passages 64 and 79.Thus, by repositioning the valve 114, the pump or pressured fluid source112 and the reservoir 110 may be connected to either of the passages toextend and retract the cylinder and piston rod assemblies, as well asthe boom sections. During the extension and retraction of the elementsof the respective fluid rams 40 and 50, the flow divider means 84 willproduce synchronized movement of the boom sections in relation to eachother.

According to a further aspect of the present invention, the flow dividermeans or valve 84 incorporates first and second pressure responsiverelief valve means 120 and 122 which produce unidirectional flow inopposite directions between the two outlet ports for interconnecting theoutlet ports when either of the fluid rams 40 or 50 is restrained frommovement whereby to maintain a synchronized movement between the fluidrams or assemblies. For this purpose, each valve assembly 120 and 122includes a valve spool 124 biased to a first position by a spring 126with the valve assembly having first and second flow passages 128 and130 therein. The valve passage 128 has a check valve 132 therein withthe valves 120 and 122 being serially arranged in a conduit 134 betweenthe first and second outlet ports 90 and 92.

Thus, in the normally biased position, the respective relief valves arelocated in the conduit 134 and respectively prevent flow from one outletto another. However, if the elements of either of the fluid rams orassemblies 40 or 50 are restrained against movement relative to eachother, the fluid pressure in the associated outlet port, for exampleport 90, will be increased and the increased pressure will be directedto the opposite end of the spool 124 through a conduit 136 and move thevalve spool to a second position where the flow passage 130 is in linewith the conduit 134. With this arrangement, all of the fluid receivedin inlet port 88 will be directed to the second outlet port 92 and causea complete extension of the elements of the second fluid ram 50. Afterthe elements of the second fluid ram are extended, the positions of thetwo valves 120 and 122 will be reversed and all of the pressured fluidfrom the inlet port 88 will be directed to the first outlet port 90 toforce extension of the elements of fluid ram 40 relative to each other.This will synchronize the fluid rams at the maximum position ofextension.

During retraction of the elements of the respective fluid rams. the flowdivider 84 will act as a flow combiner and insure equal flow from bothof the fluid rams to the inlet port. During such retraction, anyrestraint in the movement of either of the assemblies 40 or 50 willagain cause the opposite assembly to be fully retracted through theappropriate actuation of the pressure responsive valve means 120 and 122and thereafter connect both outlet ports to one assembly, the restrainedassembly, to allow that assembly to be retracted.

According to a further aspect of the present invention, the hydrauliccontrol circuit of the present invention further includes safety meansor pressure responsive relief valve means 140 for blocking flow fromeither of the fluid rams or assemblies 40 and 50 when the pressure ofthe fluid in the fluid passage means 79 is below a predetermined level.The safety valve means provide unrestricted flow to the cylinders duringextension of the respective assemblies but block the flow from thecylinders when the pressure of fluid in the retracting circuit or in thefirst passage means is below predetermined level. This arrangement willprevent uncontrolled retraction if any of the conduits should beruptured.

The valve means 140 are identical in construction and are located in theconduits 116 and 96. The valve means or valves 140, which are thecommercially available type, are schematically illustrated in FIG. 4 andinclude a passage 142 having a one way valve 144 disposed therein so asto provide substantially unrestricted flow in one direction through thevalve. This occurs during extension of the respective cylinderassemblies 40 and 50. However, before any fluid may be directed in anopposite direction through the valve, a valve element 146 in a passage148 parallel to passage 142 must be moved from its first position inwhich it blocks flow through the passage 148. This is accomplished bydirecting fluid from the respective conduits 96 and 116 throughactuating conduits 150 and 152 to the chambers 154 and thence throughthe respective valve elements 146 to the opposite end thereof to movethe valve elements or spools against the bias of a spring 156 in chamber154 and allow flow of fluid in the opposite direction through therespective safety valves. With this arrangement, there must be pressuredfluid in both conduits 96 or 116 in order for fluid to flow from thehead ends of the respective cylinders 52 and 46. Thus, forexample, ifeither of the conduits 96 or 118 were ruptured, the safety valves 140would prevent retraction of the respective boom sections relative toeach other.

From the above description it can readily be determined that thehydraulic control circuit or system of the present invention provides asimple and effective manner for extending at least three boom sectionsrelative to each other without the necessity of having moving flexibleconduits connecting the hydraulic rams of the circuit to the pressuredfluid source and the reservoir. The arrangement of the fluid cylinderand piston rod assembly in adjacent relationship on one section with thepiston rods respectively being connected to the respective sections onthe opposite ends thereof, will allow the interconnection of the twocylinder assemblies with a substantially fixed connection. Combining thecylinder assembly arrangement with a specific type of variable lengthfluid flow passage means for directing fluid to the cylinder supportingsection of the boom, substantially all of the flexible hosing mayreadily be eliminated.

The particular arrangement of the various parts forming the presenthydraulic control system, considerably reduces the cost of the hydrauliccontrol system heretofore standard in three section booms of this type.

What is claimed is:

1. In a crane having at least three extensible and retractable boomsections with a first boom section attached to a support, a hydrauliccontrol system for extending and retracting said sections comprising afirst cylinder and piston rod assembly,

with said cylinder attached to a second section and said piston rodconnected to said first section; a second cylinder and pistonrodassembly having its cylinder attached to said second section and itspiston rod attached to a third section; flow divider means on saidsecond section, said flow divider means having an inlet port, a firstoutlet port connected to one end of said first cylinder and a secondoutlet port connected to one end of said second cylinder; conduit meansinterconnecting the opposite ends of said cylinders; first flow passagemeans extending through said first cylinder and piston rod assembly andconnected to said inlet port; second flow passage means extendingthrough said first piston rod and connected to said opposite ends ofsaid cylinders; and means connecting a pressured fluid source and areservoir to said first and second flow passage means to extend andretract. said assemblies and said sections; said flow divider meansdividing the fluid received in said inlet port equally to said cylindersto produce synchronized movement of said boom sections.

2. A crane as defined in claim 1, including the further improvement offirst and second pressure responsive, unidirectional valve means betweensaid one end of said cylinders and said first flow passage means, saidvalve means being responsive to the pressure of fluid in said secondpassage means to prevent flow of fluid from said cylinders when thepressure of fluid in said first passage means is below a predeterminedlevel.

3. A crane as defined in claim 1, including the further improvement offirst and second pressure responsive relief valve means in said flowdivider valve, said valve means respectively providing unidirectionalflow in opposite directions between said outlet ports andinterconnecting said outlets when either of said assemblies isrestrained whereby to maintain said assemblies synchronized at themaximum position of extension or retraction.

4. In combination with a crane including a boom having three sectionswith a first of said sections being attached to a support, a hydrauliccontrol system comprising a source of fluid under pressure; a reservoir;first and second fluid rams each having first and second extensible andretractable elements, said first fluid ram having its first elementconnected to said first section and its second element connected to saidsecond section, said second fluid ram having its second elementconnected to said second section and its first element connected to saidthird section; a flow divider valve on said second section having aninlet port and first and second outlet ports; a first conduit connectingsaid first outlet port to one end of said second element of said firstfluid ram; a second conduit connecting said second outlet. port to oneend of said second element of said second fluid ram; a third conduitinterconnecting the opposite ends of said second elements; first fluidpassage means extending through said first fluid ram and connected tosaid inlet port; second fluid passage means extending through said firstfluid ram and communicating with said opposite end of said secondelement of said first fluid ram; and valve means for connecting saidsource and said reservoir to said fluid passage means to extend andretract said rams causing extension and retraction of said sections.

5. The combination as defined in claim 4, including the furtherimprovement of first and second relief valves, respectively, providingunidirectional flow in opposite directions between said first and secondconduits when the pressure in either of said first and second conduitsexceeds a predetermined level to provide flow to the other of saidconduits.

6. The combination as defined in claim 5, including the furtherimprovement of first safety means in said first flow passage forblocking flow when the pressure of fluid in said second fluid passage isbelow a predetermined level and second safety means in said secondconduit for blocking flow when the pressure of fluid in said thirdconduit is below a predetermined level.

1. In a crane having at least three extensible and retractable boomsections with a first boom section attached to a support, a hydrauliccontrol system for extending and retracting said sections comprising afirst cylinder and piston rod assembly, with said cylinder attached to asecond section and said piston rod connected to said first section; asecond cylinder and piston rod assembly having its cylinder attached tosaid second section and its piston rod attached to a third section; flowdivider means on said second section, said flow divider means having aninlet port, a first outlet port connected to one end of said firstcylinder and a second outlet port connected to one end of said secondcylinder; conduit means interconnecting the opposite ends of saidcylinders; first flow passage means extending through said firstcylinder and piston rod assembly and connected to said inlet port;second flow passage means extending through said first piston rod andconnected to said opposite ends of said cylinders; and means connectinga pressured fluid source and a reservoir to said first and second flowpassage means to extend and retract said assemblies and said sections;said flow divider means dividing the fluid received in said inlet portequally to said cylinders to produce synchronized movement of said boomsections.
 2. A crane as deFined in claim 1, including the furtherimprovement of first and second pressure responsive, unidirectionalvalve means between said one end of said cylinders and said first flowpassage means, said valve means being responsive to the pressure offluid in said second passage means to prevent flow of fluid from saidcylinders when the pressure of fluid in said first passage means isbelow a predetermined level.
 3. A crane as defined in claim 1, includingthe further improvement of first and second pressure responsive reliefvalve means in said flow divider valve, said valve means respectivelyproviding unidirectional flow in opposite directions between said outletports and interconnecting said outlets when either of said assemblies isrestrained whereby to maintain said assemblies synchronized at themaximum position of extension or retraction.
 4. In combination with acrane including a boom having three sections with a first of saidsections being attached to a support, a hydraulic control systemcomprising a source of fluid under pressure; a reservoir; first andsecond fluid rams each having first and second extensible andretractable elements, said first fluid ram having its first elementconnected to said first section and its second element connected to saidsecond section, said second fluid ram having its second elementconnected to said second section and its first element connected to saidthird section; a flow divider valve on said second section having aninlet port and first and second outlet ports; a first conduit connectingsaid first outlet port to one end of said second element of said firstfluid ram; a second conduit connecting said second outlet port to oneend of said second element of said second fluid ram; a third conduitinterconnecting the opposite ends of said second elements; first fluidpassage means extending through said first fluid ram and connected tosaid inlet port; second fluid passage means extending through said firstfluid ram and communicating with said opposite end of said secondelement of said first fluid ram; and valve means for connecting saidsource and said reservoir to said fluid passage means to extend andretract said rams causing extension and retraction of said sections. 5.The combination as defined in claim 4, including the further improvementof first and second relief valves, respectively, providingunidirectional flow in opposite directions between said first and secondconduits when the pressure in either of said first and second conduitsexceeds a predetermined level to provide flow to the other of saidconduits.
 6. The combination as defined in claim 5, including thefurther improvement of first safety means in said first flow passage forblocking flow when the pressure of fluid in said second fluid passage isbelow a predetermined level and second safety means in said secondconduit for blocking flow when the pressure of fluid in said thirdconduit is below a predetermined level.